1. Field of the Invention
The invention relates to a fuel injection amount control system and a fuel injection amount control device for a multi-cylinder internal combustion engine.
2. Description of Related Art
Generally, an air-fuel ratio control system including a three-way catalyst disposed in an exhaust passage of a multi-cylinder internal combustion engine and an upstream air-fuel ratio sensor located upstream of the three-way catalyst has been widely known.
The air-fuel ratio control system is configured to calculate an air-fuel ratio feedback amount (main feedback amount) based on an output value of the upstream air-fuel ratio sensor, so that the air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine (the air-fuel ratio of the engine, accordingly, the air-fuel ratio of exhaust gas) coincides with a target air-fuel ratio, and performs feedback control on the air-fuel ratio of the engine, using the main feedback amount. The feedback amount is a controlled variable common to all of the cylinders. The target air-fuel ratio is set to a given reference air-fuel ratio within the window of the three-way catalyst. Generally, the reference air-fuel ratio is the stoichiometric air-fuel ratio. The reference air-fuel ratio may be changed to a value in the vicinity of the stoichiometric air-fuel ratio, according to the intake air amount of the engine, the degree of degradation of the three-way catalyst, and so forth.
Generally, the air-fuel ratio control system as described above is applied to an internal combustion engine that employs an electronically controlled fuel injection system. The internal combustion engine has at least one fuel injection valve for each cylinder or an intake port that communicates with each cylinder. With this arrangement, if a fuel injection valve of a particular cylinder turns to “a characteristic that it injects fuel in an amount excessively larger than a designated fuel injection amount”, only the air-fuel ratio of the air-fuel mixture supplied to the particular cylinder (the air-fuel ratio of the particular cylinder) changes largely into a richer (smaller) value. Namely, the degree of ununiformity in the air-fuel ratio among cylinders (variations in the air-fuel ratio among cylinders, cylinder-to-cylinder air-fuel ratio imbalance proportion) becomes larger. In other words, significant imbalances appear among “the air-fuel ratios of the individual cylinders” as the air-fuel ratios of air-fuel mixtures supplied to the respective cylinders. Then, the degree of ununiformity in the air-fuel ratio among cylinders becomes larger.
In the following description, the cylinder corresponding to “the fuel injection valve having a characteristic of injecting fuel in an amount excessively larger or excessively smaller than the designated fuel injection amount” will be called “imbalance cylinder”, and the remaining cylinders (cylinders corresponding to “fuel injection valves that inject fuel in the designated fuel injection amount”) will be called “non-imbalance cylinders (or normal cylinders)”.
If the fuel injection valve of a certain particular cylinder turns to “a characteristic that it injects fuel in an amount excessively larger than the designated fuel injection amount”, the average of the air-fuel ratios of air-fuel mixtures supplied to the engine as a whole becomes a richer air-fuel ratio than the target air-fuel ratio set to the reference air-fuel ratio. Accordingly, with the feedback amount of the air-fuel ratio common to all the cylinders, the air-fuel ratio of the particular cylinder is changed to a leaner (or larger) value so as to be closer to the reference air-fuel ratio, and at the same time, the air-fuel ratio of the other cylinders is changed to a leaner (or larger) value, deviate further from the reference air-fuel ratio. As a result, the average of the air-fuel ratios of the mixtures supplied to the engine as a whole (the average air-fuel ratio of exhaust gas) becomes equal to an air-fuel ratio in the vicinity of the reference air-fuel ratio.
However, the air-fuel ratio of the above-indicated particular cylinder is still a richer air-fuel ratio than the reference air-fuel ratio, and the air-fuel ratio of the remaining cylinders becomes a leaner air-fuel ratio than the reference air-fuel ratio. As a result, the amount of emissions discharged from each cylinder (the amount of unburned substances and/or the amount of nitrogen oxides) is increased, as compared with the case where the air-fuel ratio of each cylinder is equal to the reference air-fuel ratio. Therefore, even when the average of the air-fuel ratios of the mixtures supplied to the engine is equal to the reference air-fuel ratio, the increased emissions cannot be completely cleaned by the three-way catalyst, which may result in deterioration of the emissions.
In order to prevent emissions from being deteriorated, therefore, it is important to detect excessively large ununiformity in the air-fuel ratio among the individual cylinders (namely, the occurrence of air-fuel ratio imbalances among the cylinders), and take some measure against the imbalance condition. The air-fuel ratio imbalances among cylinders also occur, for example, when the fuel injection valve of a particular cylinder turns to “a characteristic that it injects fuel in an amount that is excessively smaller than the designated fuel injection amount”.
One example of fuel injection amount control system according to the related art obtains a trace length of an output value (output signal) of an electromotive force type oxygen concentration sensor located upstream of a three-way catalyst. The control system compares the trace length with “a reference value that varies according to the engine speed”, and determines whether an air-fuel ratio imbalance condition among cylinders has occurred based on the result of comparison (see, for example, U.S. Pat. No. 7,152,594).
In the meantime, the exhaust gases emitted from the respective cylinders reach the upstream air-fuel ratio sensor in the order in which the ignition takes place in the cylinder (which is the same as the order in which exhaust gas is discharged from the cylinder). If there is no difference in the air-fuel ratio among the cylinders (i.e., if no imbalance in the air-fuel ratio among the cylinders occurs), the air-fuel ratio of exhaust gas emitted from each cylinder and reaching the upstream air-fuel ratio sensor is substantially equal. Accordingly, when there is no difference in the air-fuel ratio among the cylinders, the output value Vabyfs of the upstream air-fuel ratio sensor varies as indicated by the broken line C1 in FIG. 3B, for example. Namely, if there is no imbalance or ununiformity in the air-fuel ratio among the cylinders, the output value Vabyfs of the upstream air-fuel ratio sensor assumes a generally flat waveform pattern. Therefore, the trace length of the output value Vabyfs of the upstream air-fuel ratio sensor is short. Also, the absolute value of the differential value d(Vabyfs)/dt (rate of change ΔAF) of the output value Vabyfs is small, as indicated by the broken line C3 in FIG. 3C.
On the other hand, if “the fuel injection valve that injects fuel into a particular cylinder (e.g., first cylinder)” is provided with “a characteristic that it injects a larger amount of fuel than the designated fuel injection amount”, a difference in the air-fuel ratio among the cylinders becomes large. Namely, there arises a large difference between the air-fuel ratio of exhaust gas of the particular cylinder (the air-fuel ratio of the imbalance cylinder), and the air-fuel ratio of exhaust gases of the cylinders other than the particular cylinder (the air-fuel ratio of the non-imbalance cylinders).
Accordingly, when the degree of ununiformity in the air-fuel ratio among the cylinders becomes large, the output value Vabyfs largely fluctuates in a cycle of a unit combustion cycle period, as indicated by the solid line C2 in FIG. 3B, for example. Therefore, the trace length of the output value Vabyfs of the upstream air-fuel ratio sensor is increased. Also, when the degree of ununiformity in the air-fuel ratio among the cylinders becomes large, the absolute value of the differential value d(Vabyfs)/dt (the change rate ΔAF) becomes large, as indicated by the solid line C4 in FIG. 3C. In this connection, the unit combustion cycle period is a period it takes the engine to rotate by a crank angle required for all of the cylinders from which exhaust gases reaching the upstream air-fuel ratio sensor are emitted, to complete one combustion cycle of each cylinder.
Furthermore, the above-indicated trace length increases and the absolute value |ΔAF| of the change rate ΔAF fluctuates by a larger degree, as the air-fuel ratio of the imbalance cylinder deviates further (by a larger degree) from the air-fuel ratio of the non-imbalance cylinders. For example, if the output value Vabyfs varies as indicated by the solid line C2 in FIG. 3B when a difference between the air-fuel ratio of the imbalance cylinder and the air-fuel ratio of the non-imbalance cylinders is a first value, the output value Vabyfs varies as indicated by the one-dot chain line C2a in FIG. 3B when a difference between the air-fuel ratio of the imbalance cylinder and the air-fuel ratio of the non-imbalance cylinders is “a second value that is larger than the first value”.
As is understood from the above description, the fluctuations in the output value Vabyfs of the upstream air-fuel ratio sensor become larger as the degree of ununiformity in the air-fuel ratio among the cylinders is larger; therefore, an air-fuel ratio imbalance index value representing the degree of ununiformity in the air-fuel ratio among the cylinders can be acquired based on the output value Vabyfs.
In the meantime, when the engine 10 is in a transient operating condition, such as an accelerating condition or a decelerating condition, for example, the center of the air-fuel ratio (center air-fuel ratio) of the engine may largely vary at a relatively low frequency (see the broken line L1 of FIG. 4). Such variations in the center air-fuel ratio appear in the output value Vabyfs of the upstream air-fuel ratio sensor. Accordingly, the output value Vabyfs of the upstream air-fuel ratio sensor is in the form of a signal (see the solid line L3 of FIG. 5) obtained by superimposing a signal (see the solid line L2 of FIG. 4) indicative of fluctuations in the air-fuel ratio due to the ununiformity (or imbalance) in the air-fuel ratio among the cylinders, on a signal (see the broke line L1 of FIG. 4 and FIG. 5) indicative of fluctuations in the center air-fuel ratio of the engine.
In this case, the output value Vabyfs of the upstream air-fuel ratio sensor fluctuates largely even if the degree of ununiformity in the air-fuel ratio among the cylinders is small. For example, when the air-fuel ratio imbalance index value is calculated based on the differential value d(Vabyfs)/dt (the rate of change ΔAF, or slope) of the output value Vabyfs, the slope obtained when the center air-fuel ratio of the engine does not fluctuate is θ1 as shown in FIG. 4, whereas the slope obtained when the center air-fuel ratio of the engine fluctuates is θ2 (θ2>θ1) as shown in FIG. 5 even with the same degree of ununiformity in the air-fuel ratio among the cylinders. Consequently, if an air-fuel ratio imbalance index value is obtained based on the output value Vabyfs of the upstream air-fuel ratio sensor, the obtained air-fuel ratio imbalance index value may not accurately represent the degree of ununiformity in the air-fuel ratio among the cylinders.
Therefore, if it is determined based on the air-fuel ratio imbalance index value obtained as described above whether an imbalance condition in the air-fuel ratio among the cylinders occurs, an erroneous determination may be made. If “fuel amount increasing control for compensating for erroneous lean correction which will be described later” is performed based on the air-fuel ratio imbalance index value as described above, the air-fuel ratio of the engine may not be controlled to an appropriate air-fuel ratio.